I saw your post in "Tricks of the Trade" as well and you've provided so additional information in this post.

I would consider this to be a high risk situation. You have gas (compressed air) under static pressure so that if an explosion occurs the pipe is likely to rupture. The air is under pressure but apparently not flowing, so there is no cooling effects as there would be with liquid or gas that is flowing at a high rate. The compressed air contains oxygen so it will support combustion unlike the situation with a gas or liquid petrochemical line where there is no oxygen available to support combustion.

Are you asking for the advisability of your application? If so, my advice is, "don't do it."

There must be a way to open the pipe so that pressure is not confined if the residue does ignite. Better still, fill the pipe with CO2 so there is no danger of ignition and have a  small "bleed off" so there is not a build up of pressure while welding. You can use CO2 gas from a high pressure cylinder used to provide shielding gas for GMAW with the appropriate pressure reducing flow meter. The CO2 is relatively inexpensive and eliminates the possibility of an explosion or ignition of the oily residue in the pipe. You would set it up as if you were purging the pipe for welding.

Best regards - Al

If you are planning to do a hot tap with the line in service and your main concern is the ignition of the oil residue in the pipe, you have absolutly no idea what you are doing. If it is your objective to do this work while the line is in service, PLEASE hire someone who knows the answer to your question and can write the procedure on what to do to give instruction to the welder. If you are using a contractir, please have a contractor who has done hot taps in the past. But even if they have done the hot taps, YOU are responsible for the procedures to give to the contractor.

You could get someone hurt!

If you are going to do this while in service, you will have to keep the air moving. Welding on the pipe while it is pressurized and the air is not moving is a recepie for disaster. The ignition of the residue would be the least of your worries. If you are talking about shutting the line down, cutting the line  and dropping in a tee, then that is not a hot tap. If you are going to weld on the line while it is pressurized, keep the air moving and use small diameter 7018 electrodes. Also secure API publication 2201 Procedures for Welding or Hot Tapping on Equipment In Service This covers the procedures and requirements for this type of work.
BABRT's

Call these folks. They are who I use. Hot Tap on a 90 psi commpressed air line is not that big of a deal. They will do you a great job.

Jim

www.topazinc.com

why do you not just de pressurize the pipe?
if the concern is cost of shutting down production the consider the cost if things go wrong.
what is the pipe made of?

in regard to your concern about gases exploding perhaps this will shed some light on the possibility
Incident Bulletin
Forestry
Welding on assembled wheel results in explosion

A stacker wheel with less than 4,500 hours of use developed a slow air leak in a weld along the circumference of the rim. The factory weld had been weld-repaired three times before in the previous two months.

As with previous repairs, the worker removed the valve core and removed most of the air from the tire before welding. This time the weld was approximately 1 x 6 inches on the rim. The tire was still mounted. After completing the weld, the worker attached an air line and was adding air when the tire exploded. The worker died instantly.

After the explosion, investigators found there had been a fire inside the tire. It is believed welding heated the nearby rubber, which then started to smoulder and released flammable gas. When air was added to inflate the tire, it fanned the smouldering area into a fire. When the ratio of flammable gas and oxygen reached the explosive limit, the tire exploded.

Show image

What should be done to prevent a similar incident

    * Do not weld on an assembled wheel. Remove the tire first. Then weld only if allowed by the manufacturer or under the direction of an engineer.
    * Consider inflating the tire with nitrogen instead of air. Use a source regulated to the maximum pressure permitted for the tire being filled.
    * Work on assembled wheels must be conducted in compliance with the Occupational Health and Safety Regulation.



Relevant sections of the Occupational Health and Safety Regulation*

16.48 Equipment and procedures

...

(7) Welding or heating on assembled rim or wheel parts is not permitted, except that limited heating to facilitate removal of a wheel from a hub is acceptable after the tire has been deflated by removing the valve core.

...

____________

*The section of the Occupational Health and Safety Regulation listed in this bulletin does not imply a breach of the Regulation on the part of any party that may have been involved with this incident. The purpose in listing specific sections of the Regulation is to make readers aware of some of the applicable sections of the Regulation.

this scenario is also quite possible with the oil and residue inside the pipe
any way you look at it its gonna cost some dough to get it done, your main concern should be that it does not cost a life
darren

Additionnal informations about the application:

There is air flow in the line (volume not known yet). Shall be set at maximum.

We know API 2201.

We will measure cooling rate and control heat input.

We have done hot tap in natural gas and oil industry.

We know how to measure inside pipe temperature during welding. (To be done before hot tap).

Main concern is about minimum inside pipe temperature for ignition.

Thanks for helping me

Regards,

Yvan
Montréal
Canada

Thanhs for helping me with that work.

Regards

Yvan
Montréal
Canada
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We know it has already been done.

However we're not gone do it before we confirm no ignition risk

Yvan
Montréal
Canada